In terms of coercivity (Hc), product of remanence and film thickness (Mr S), coercivity squareness (S*), media signal-to-noise ratio (SNR) and off-track capability (OTC), it is difficult to make films satisfying all such requirements, especially for the SNR and OTC, in the high linear density regime.
Linear recording density can be increased by increasing the media coercivity, reducing the product of remanence and film thickness (Mrt) and reducing the head flying height which requires flatter and smoother media substrates. However, the media noise has been found to increase dramatically with linear density. Therefore, the media noise has become a road barrier to ultra-high areal density recording. Media noise, originating from exchange coupling among magnetic grains, is a dominant factor restricting increased recording density of high density magnetic hard disk drives. Studies have shown that media noise is attributed primarily to intergranular exchange coupling and larger distribution of medium magnetic grain size, but little has been emphasized on the morphology of the media and topography of the film surface. As the areal recording density increases and as magnetoresistance recording (MR) technology is employed, media Mrt reduces and magnetic film thickness decreases, thereby increasing the significance of morphology and topography effects on media performance. In order to increase linear density, one has to grow films in a controlled manner to obtain a suitable microstructure, i.e., crystallographic orientation, grain size, morphology and topography, therefore minimizing media noise.
A typical longitudinal recording medium is depicted in FIG. 1 and comprises a substrate 10, typically an aluminum (Al)-alloy, such as an aluminum-magnesium (Al--Mg)-alloy, plated with a layer of amorphous nickel-phosphorous (NiP). Alternative substrates include glass, glass-ceramic materials and graphite. Substrate 10 typically contains sequentially deposited on each side thereof a chromium (Cr) or Cr-alloy underlayer 11, 11', a cobalt (Co)-base alloy magnetic layer 12, 12', a protective overcoat 13, 13', typically containing carbon, and a lubricant topcoat 14, 14'. Cr underlayer 11, 11' can be applied as a composite comprising a plurality of sub-underlayers 11A, 11A'. Cr underlayer 11, 11', Co-base alloy magnetic layer 12, 12' and protective carbon overcoat 13, 13' are usually deposited by sputtering techniques performed in an apparatus containing sequential deposition chambers. A conventional Al-alloy substrate is provided with a NiP plating, primarily to increase the hardness of the Al substrate, serving as a suitable surface for polishing to provide a texture, which is substantially reproduced on the disk surface.
As previously mentioned, another important approach is to lower the recording head flying height or minimize the physical spacing between the head and media. This requires the surface of the substrate to be flatter and smoother. The requisite spacing has approached less than one micron. In this regime, the media topography and morphology have become increasingly more important. As thin film recording technology rapidly evolves, non-magnetic substrates are being fabricated with smoother topography and magnetic layers of decreasing thickness. The flying height or gap between recording head and media decreases, thereby exacerbating the impact of film morphology on film growth and the preparation of the media. Accordingly, there exists a need to produce magnetic recording media from a basic film growth view point, to grow films with better morphology, homogeneity and film integrity in addition to satisfying all the other crystallographic lattice match requirements for high areal density, and for an efficient method of manufacturing a high signal-to-noise ratio (SNR) magnetic recording medium.